This invention relates to the disposal of spent potlining by incineration and glassification for landfill disposal.
Aluminum is produced commercially by the electrolytic smelting of alumina in an electrolytic bath of fused cryolite. Relatively pure alumina is reduced in the cryolite (3NaF--AlF.sub.3) in an electrolytic cell (named Hall-Heroult cell after the earliest developers of the electrolytic process for producing aluminum). The reaction is carried out in the Hall-Heroult cell process in an aluminum reduction pot in which alumina is dissolved in the molten cryolite bath. The alumina in solution in the molten cryolite is electrolyzed to form metallic aluminum. Aluminum produced in the reaction is heavier than the electrolyte and forms a molten layer at the bottom of the reduction pot which serves as the cathode of the cell. Carbon anodes extend into the bath, and oxygen liberated at the anode oxidizes carbon on the electrode.
The electrolyte composition is an important factor in the aluminum production process. The electrolyte contains cryolite and fluorspar (calcium fluoride, CaF.sub.2). Excess aluminum fluoride is present with dissolved alumina in the electrolyte to reduce the liquidus temperature to the point where the cell can be operated in the range of about 940.degree.-980.degree. C.
Gases are emitted by the batteries of reduction pots in the Hall-Heroult cell process. The emitted gases include carbon dioxide, volatilized fluorides, and gaseous hydrogen fluoride. Particulate solids, including fine alumina, cryolite, carbon, and inorganic compounds are elutriated from the electrolytic bath with the gases. Small amounts of lithium, calcium, silica, iron, sodium sulfate, and magnesium are present also in the gas streams as solid particles.
The reduction cells in the commercial electrolytic potlines are lined with a carbonaceous material. During the life of the cells, the carbonaceous material linings are degraded gradually by penetration of bath materials into the lining, e.g., such as by penetration by metallic aluminum, cryolite, and alumina. In the high temperatures of the electrolytic reduction process, the carbonaceous material ages gradually and degrades over time. Eventually, the unusable or "spent" potlining must be removed and discarded.
It is known that spent potlining from aluminum reduction cell linings contains a significant quantity of carbonaceous material, a mixture of fluoride salts, some low levels of cyanide, and alumina (Al.sub.2 O.sub.3).
The Environmental Protection Agency is considering classifying spent potlining from the Hall-Heroult cell as a solid waste hazardous material because of the hazardous and solid waste amendments of 1984. Most spent potlining cannot be landfilled because of cyanide content (approximately 1,000 parts per million) and because of its high content of leachable fluoride (3,000 to 8,000 parts per million) from an approximately 18% total fluoride in the spent potlining.
Several methods already have been recommended for the recovery of fluoride salts and alumina content of the spent potlining material. One method involves the pyrohydrolysis of the spent cell material or potlining in a fluidized bed reactor. Pyrohydrolysis involves contacting a fluidized bed of spent potlining with water or steam to react with the fluorine compounds to form hydrogen fluoride. Sodium fluoride and other sodium compounds in the potlining react to form sodium fluoride and sodium oxide vapors. The sodium fluoride and sodium oxide vapors are produced in the pyrohydrolysis reactions.
It is known to use limestone, calcium carbonate, to react with fluoride in the spent potlining at about 700.degree.-780.degree. C. to form a calcium fluoride. However, the final product still has a high level of leachable fluoride. An article in Light Metals, 1981, entitled "Recovery of Fluoride and Fluorine Balance of Aluminum Smelting Plant" describes a process of spent potlining wherein the potlining is crushed, incinerated, and introduced into a crusher for pulverizing and extraction of the fluorides into water. After filtration, the material is introduced into a mixing tank, and sulfuric acid and calcium carbonate are added to produce calcium fluoride. The slurry is desiccated, and the sludge sent to a disposal site. The fluoride-containing filtrate is used as the scrubbing solution in a wet scrubber.
U.S. Pat. No. 4,113,832 discloses a process wherein spent potlining is leached with a caustic solution followed by a precipitation of sodium fluoride by saturating the leach liquor with a compound to suppress the solubility of the sodium fluoride in the leach liquor. The preferred compound is ammonia. The precipitated sodium fluoride is removed, and the leach solution saturated with ammonia is processed to remove the ammonia.
U.S. Pat. No. 4,444,740 discloses a process for removing and recovering fluoride values from spent potlining by leaching with a dilute caustic. The leachate is treated with a calcium compound to precipitate calcium fluoride.
The conventional methods for removing and recovering fluorides from spent potlining are impractical ways to avoid the problem of spent potlining disposal with fluorides and cyanides present. Prior methods adopt the approach of removing and recovering the fluoride. These prior art methods are impractical by reason of the raw material costs assigned to the process and also by reason of the process steps which must be carried out to remove and recover the fluorides.
U.S. Pat. No. 4,735,784 to Davis et al (currently assigned to the Morrison-Knudsen Company, Inc.) discloses a process for treating solid, substantially non-volatile waste contaminated with a heat-sensitive contaminant by decomposing or evolving the contaminants from the melt as a gas. Davis et al disclose a molten slag subjected to cooling, the contaminating compounds bound or encapsulated into a solid glassy slag, and suitable as such for handling fluoride and cyanide contaminated aluminum smelting wastes such as spent potliner material. Silica can be added to such wastes. The Davis et al process produces significant volumes of fluoride emissions. The patent covers a pyrohydrolysis process, disclosed and claimed. The process forms a slag in the presence of sufficient water for pyrohydrolysis conditions resulting in the volatilization of substantially all of the fluoride contaminants, mostly in the form of hydrogen fluoride.
The Davis et al process includes a single heating step in which the mixture of spent potlining and silicates is heated to 1000.degree.-1700.degree. C. Davis et al heat the mixture to the temperature range of 1000.degree.-1700.degree. C. to volatilize substantially all the fluoride in the spent potlining. Steam is used in the process to provide hydrogen to combine with the fluoride to produce hydrogen fluoride. Davis et al trap some fluoride in the glass residue, but only a minor portion of the total fluoride from the spent potlining. As stated in Column 3, lines 30-33, and in the claims of Davis et al, substantially all the fluoride contaminants are volatilized.
It is an object of the present invention to provide a process for eliminating the problem of disposal of spent potlining containing fluorides in such a way to reduce gaseous emissions of fluorine or fluoride in any form.
It is an object of the present invention to provide a practical method for eliminating the problem of disposal of spent potlining containing fluorides.
It is another object of the present invention to provide a practical method for the disposal of spent potlining which reduces degradation of the environment.
These and other objects of the present invention will become apparent from the detailed description which follows.